1. The Field of the Invention
The present invention relates generally to chemical-mechanical polishing (CMP) of a semiconductor substrate. In particular, the present invention relates to improving the wetting capability of polishing solutions for fixed-abrasive CMP of hydrophobic surfaces on a semiconductor substrate without compromising the chemical action of the polishing solution. The present invention also comprises a CMP pad that mechanically draws or forces polishing solution between a hydrophobic surface to be polished and a hydrophobic fixed-abrasive polishing pad.
2. The Relevant Technology
In the microelectronics industry, a substrate refers to one or more semiconductor layers or structures which includes active or operable portions of semiconductor devices. In the context of this document, the term "semiconductor substrate" is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term "substrate" refers to any supporting structure including but not limited to the semiconductor substrates described above. A semiconductor device refers to a semiconductor substrate upon which at least one microelectronic device has been or is being batch fabricated.
In conventional CMP technology a slurry is distributed between a resilient pad and the surface to be polished. In conventional slurried CMP technology, the surface tension of the liquid is not of great concern because slurry particulates have a trajectory within the polishing area, such that the particulates will impact the surface, regardless of the hydrophobicity of the surface to be polished and the surface tension of the polishing liquid.
In a conventional CMP apparatus, a semiconductor substrate to be polished is mounted on a polishing block which is placed on the CMP machine. A polishing pad is adapted to engage the semiconductor substrate carried by the polishing block. A cleaning agent is dripped onto the pad continuously during the polishing operation while pressure is applied to the semiconductor substrate.
A typical CMP apparatus comprises a rotatable polishing platen and a polishing pad mounted on the platen. Platen and pad are typically driven by a microprocessor controlled motor to spin at about 0 to about 200 RPM. A semiconductor substrate is mounted on a rotatable polishing head so that a major surface of the semiconductor substrate to be polished is positionable to contact the polishing pad. The semiconductor substrate and polishing head are attached to a vertical spindle that is rotatably mounted in a lateral robotic arm that rotates the polishing head at about 0 to about 50 RPM in the same direction as the platen and radially positions the polishing head. The robotic arm also vertically positions the polishing head to bring the semiconductor substrate into contact with the polishing head and maintain an appropriate polishing contact pressure
A tube opposite the polishing head above the polishing pad dispenses and evenly saturates the pad with an appropriate cleaning agent, typically a slurry. The slurry-assisted polishing pad is typically porous, which favors wetting of the polishing surface.
Other CMP techniques include orbiting or oscillating motions of either the article to be polished or of the polishing pad, or both. Other CMP techniques include a belt-shaped polishing pad that is advanced translationally under the article to be polished, and the article to be polished is rotated, oscillated, or both across the surface of the belt-shaped pad.
In fixed-abrasive CMP technology, a polishing solution is distributed between a resilient resin pad containing abrasives and the surface to be polished. The pad can be made from substances that are hydrophobic. These substances include amines, organic polymers, and resins. In conventional polishing of oxide surfaces the aqueous polishing solution sufficiently wets the oxide surface because water is also an oxide and the surface tension between the two is sufficiently low that the solution wets the oxide surface.
CMP of hydrophobic surfaces includes substances such as monocrystalline silicon, HSG silicon, amorphous silicon, polycrystalline silicon (polysilicon), silicides such as tungsten and titanium silicide, interlayer dielectrics such as PTFE and refractory pure metals or alloys such as tungsten, titanium, and copper.
Conventional CMP of hydrophobic surfaces with fixed-abrasive pads that are likewise hydrophobic presents a challenge to keep a uniformly-wetted surface where polishing is done with an aqueous solution. Between the two hydrophobic surfaces of the fixed-abrasive pad and the surface to be polished, there exists no surface that wets easily. This resistance to wetting hinders uniform coverage of the polishing solution. Attempting to force an aqueous polishing solution between two hydrophobic surfaces results in the formation of aqueous solution beads at the perimeter of the pad and no chemical action occurs. With no chemical action, polishing is ineffective and CMP fails. The result is that the surface to be polished is scratched and the semiconductor substrate is damaged or destroyed.
In the chemical makeup of the polishing solution for hydrophobic semiconductor surfaces, two factors of sufficient wetting and sufficient chemical action are required. In fixed-abrasive CMP of hydrophobic surfaces, sufficient chemical action requires a balance between sufficient chemical polishing and sufficient chemical selectivity that achieves both CMP of hydrophobic surfaces and stopping on nonhydrophobic surfaces. Additionally, where CMP is carried out within a single film, although chemical selectivity is not an issue, there remains the requirement of achieving sufficient wetting and sufficient chemical action.
FIG. 1 depicts the wetting of a polishing solution on a surface to be polished. In the droplet of moisture, an angle known as .theta., or the contact angle, forms between the plane of the solid surface to be wetted and the slope of the liquid contacting the solid surface. In describing the forces at a solid-liquid-gas interface 12, three surface tensions must balance in a static situation. The surface tension between the solid and the gas, .gamma..sub.sg, is usually very small. In FIG. 1 the surface tension of the solid and gas, .gamma..sub.sg, is depicted as a vector 14 at the solid-liquid-gas triple point. The surface tension of the solid and liquid, .gamma..sub.sl, is depicted as a vector 16 at the triple point. The surface tension of the liquid and the gas, .gamma..sub.lg, is depicted as a vector 18 that forms an angle, .theta. with the solid surface. A force balance around the triple point reveals that EQU .gamma..sub.sl =.gamma..sub.lg cos .theta.+.gamma..sub.sg (1)
This expression can be rearranged to be solved for the contact angle .theta. as EQU cos .theta.=(.gamma..sub.sl -.gamma..sub.sg)/.gamma..sub.lg. (2)
FIG. 2 illustrates the interplay between surface tension of the liquid in the gas and surface tension of the solid in the liquid where the surface tension of the solid is held constant. If the surface tension of the liquid in the gas is high, an acute angle, .theta..sub.1 is formed and the surface of the solid is called hydrophobic. If the surface tension of the solid in the liquid exactly equals the surface tension of the solid in the gas then the contact angle is a right angle, .theta..sub.2 and the surface of the solid is neutral to hydrophobicity or hydrophilicity. If the surface tension of the liquid in the gas is low enough an obtuse angle .theta..sub.3 is formed and the surface of the solid is called hydrophilic. Equation 2 does not hold, however when complete wetting occurs such that .theta..sub.3 is 180 degrees and .gamma..sub.sg &gt;.gamma..sub.sl +.gamma..sub.lg, or for no wetting at all such that .theta..sub.1 is zero degrees and .gamma..sub.sl &gt;.gamma..sub.sg +.gamma..sub.lg.
FIG. 3 illustrates the inadequate wetting problem of the prior art. In FIG. 3 a semiconductor substrate 200 has been patterned and etched through an oxide or nitride layer 202 to form a trench or hole 204 in a silicon substrate 206. Upon oxide or nitride layer 202 a polysilicon layer 208 is deposited that fills trench or hole 204 and covers the entire upper surface of oxide or nitride layer 202. To form a contact, polysilicon layer 208 is illustrated as being polished with a fixed-abrasive CMP pad 210 and the surface is being wetted with a polishing solution 112. Due to the hydrophobicity of both pad 210 and polysilicon layer 208 polishing solution 112 forms acute contact angles at the edge of pad 210 and polishing solution 112 is not drawn under pad 210 such that the chemical aspect of CMP is not accomplished.
FIG. 4 depicts section 4--4 taken from FIG. 3 in which a closer view of failed wetting of the polishing solution on a hydrophobic surface is illustrated. In FIG. 4 it is illustrated that the contact angle .theta. is acute such that polishing solution 112 is not drawn under pad 210. Because polishing solution 112 is not drawn under pad 210, wetting does not occur between pad 210 and polysilicon layer 208, and therefore CMP is not accomplished.
What is needed is a polishing solution, in combination with chemical polishing parameters, that wets either the fixed-abrasive pad or the polishing surface sufficiently to activate CMP without altering the necessary chemical composition of the polishing solution to the point that it no longer serves its role in the chemical portion of CMP. What is alternatively needed is a fixed-abrasive pad that, although flexible and resilient, is physically configured such that wetting across the pad is sufficient to transfer the polishing solution uniformly across the surface to be polished to activate the entire CMP process.
In connection with a polishing solution that will uniformly wet a hydrophobic surface to be polished, what is also needed is a polishing solution that will not continue its CMP action if the surface were one where it is effaced down to a hydrophilic surface such as an oxide.